Interesting...this doesn't happen for me.If the alarm is activated and you use the trunk button on the remote, you have about 5-10 seconds to open the trunk without the alarm sounding. If you wait too long, it will go off when you raise the lid.
Now having the alarm go off 30 seconds after you raise the lid is rather strange - I've never had that happen. I have seen it happen that I thought the trunk was closed but it wasn't, and the alarm sounds maybe 30 seconds after I lock the car. But I generally get in the habit of unlocking the car before I open the trunk because I can never be sure what it's going to do.